1. Field of the Disclosure
The subject disclosure relates to communication in sensing, monitoring and control systems, and more particularly to synchronizing wireless devices used on board aircraft.
2. Background of the Related Art
In recent years, the aerospace industry has been actively working on using wireless communication as a replacement for wired data connections in aircraft systems. Some benefits of wireless as opposed to traditionally wired sensor include weight and complexity reductions, simplified installation and maintenance, easier system reconfiguration as well as better diagnostic analysis. Additionally, new sensing technologies and aircraft subsystems are enabled by utilizing wireless technology, which eliminates additional design and installation costs and limits the additional wiring weight.
Mechanical and structural health monitoring and diagnostic systems particularly benefit from wireless technology. At present, wider use of such systems is hampered by the increased cost and complexity of the required wiring, which can often outweigh the potential gains. If no additional wiring were necessary, such systems might be more readily used, which would lead to significant improvements in maintenance costs and in aircraft reliability.
Most sensing and actuation systems require some degree of synchronicity between different elements. For example, a mechanical diagnostic system may collect vibration data from multiple sensors. For correct interpretation of multi-dimensional vibration data, it is necessary to make sure that the acquired signals correspond to the same intervals in time. The synchronization is typically achieved in one of two ways. The first approach is to devise a method to make sure that all sensors start their data acquisition at the same time. The second approach is to have all sensor nodes measure their local time according to well synchronized clocks.
The two approaches to synchronization are in fact equivalent. If it is possible to command several sensor nodes to perform a certain task starting at the same time, then the task may involve starting or resetting their clocks. If clock resets are done simultaneously, then the clocks will be closely synchronized for some time provided their rates do not differ too much. On the other hand, if the clocks are tightly synchronized, then the sensors may be commanded to perform the task of interest, such as to start data acquisition, at particular time values according to their clocks. In other words, the respective sensor's reading can be correlated in such way that the readings correspond to the same physical time instance.
Several techniques for synchronization of wireless devices are known and typically involve exchanges of time-stamped messages. Based on known send and receive times of sent messages, offsets and rate differences may be estimated between the clocks of those devices. For example, see “Protocols and architectures for wireless sensor networks” by Holger Karl and Andreas Willig, (published by John Wiley & Sons, Ltd. in the year 2005, hereinafter the Karl/Willig approach), which is incorporated herein by reference.
The Karl/Willig approach rests on the assumption that propagation and processing delays of the messages are symmetric and statistically constant in time. That is, it is assumed that any random delays in delivering a message from wireless node N1 to the wireless node N2 are distributed in the same way as message delivery delays from wireless node N2 to wireless node N1, and this statistical distribution is stationary in time. Furthermore, the accuracy of the resulting clock synchronization depends on the variability of message delivery delays. Generally, as the variance of message delivery delays gets larger, so does the synchronization error. Therefore, for synchronization to be accurate in the Karl/Willig approach, it is desirable to have as little variance in message delivery delays as possible.
In an effort to reduce development costs and to leverage the huge level of investment in consumer wireless technology, it is desirable to build wireless aircraft systems using commercial off-the-shelf (COTS) wireless components. COTS products typically do not allow access to lower level protocol functions but instead interface with the sensing device above a high-level network protocol. For example, wireless devices using the standard 802.11 protocol may be commercially available in form of modules allowing communication via TCP or UDP protocols.
Use of high level protocols is desirable because the software development costs are reduced and communication reliability is increased. The TCP protocol particularly represents years of development of reliable network communication and includes many built-in mechanisms to ensure dependable delivery of large quantities of data. In contrast, the much simpler UDP protocol does not include such reliability mechanisms, so the communication link may experience high and unpredictable message losses. For this reason, from the software development point of view, the use of TCP may be more attractive than UDP, because TCP reduces or eliminates the need to add customized reliability mechanisms.